Summary Posttraumatic stress disorder (PTSD) is a highly distressing and potentially disabling condition involving altered fear learning that occurs in some individuals after exposure to highly traumatic events. While generic antidepressants and short-term anxiolytics are the mainstays of PTSD treatment, these drugs remain insufficient to treat the disease. Thus, there remains a large unmet medical need to develop novel therapies for PTSD and other fear related disorders. In our previously published work, in a cohort of 1847 human PTSD patients with a history of significant lifetime trauma, we found that a single-nucleotide polymorphism (SNP) within the gene encoding the nociceptin receptor (NOP-R) is associated with increased PTSD symptoms. We also found that this SNP is associated with fear generalization and increased amygdala-insula functional connectivity. Using a mouse model of dysregulated fear, we found altered expression of the oprl1 gene (encoding NOP-R) within the amygdala. Systemic and central amygdala infusion of SR-8993, our highly selective NOP-R agonist, impaired fear memory consolidation, suggesting that NOP-R is associated with fear processing and PTSD symptoms. Our data also suggest that activation of the NOP-R interferes with fear memory consolidation, with implications for the treatment of PTSD even after a traumatic event. The specific goal of this project is to develop useful NOP-R modulators to understand the biology of the NOP-R with a view toward eventual clinical application. The compounds we developed in previous efforts, and their analogs, are best-suited to reach this goal due to unprecedented selectivity for the NOP-R over the other opioid receptors, particularly the mu opioid receptor (MOR). Selectivity for NOP-R over the mu receptor is necessary to reduce addiction potential and avoid the unwanted side effects that are associated with MOR activation, including respiratory depression, constipation and itch/pruritus. However, our published lead compound, SR-8993, with some selectivity for NOP-R over MOR, continues to have significant MOR agonist activity, one feature that we aim to correct in this proposed work. In support of the feasibility of our approach, we show unpublished compounds that have over 10,000-fold selectivity for NOP-R over MOR. Additional chemistry efforts will be aimed at improving the drug-like characteristics of preferred leads and to improve pharmacokinetic (PK) properties (Aim 1). Pharmacological profiling will include assessments of NOP-R and opioid receptor function with further assessments of DMPK parameters to ensure the potential for in vivo utility (Aim 2). PTSD-like animal models will be used to assess target-exposure relationships, and a battery of in vivo neuro-behavioral and also addiction potential assessments will be used to determine potential for undesired effects (Aim 3). We aim to deliver a set of pre-clinically validated, safe, highly selective, brain penetrant small molecule nociceptin receptor agonists that attenuate PTSD-like fear and anxiety behavior in well-validated PTSD models that are primed for clinical translation.